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Volt charging from empty to full in 3:30 hours last night:
This is probably the only thing that surprises me. Usually from completely empty to a full charge takes over 4 hours.

Every hour there is a short 1-minute period where the charger uses only half of the rated power. Why is that? Is it to re-calibrate the voltages in the battery management systems without a charge current? It's not clear to me. This power dip is always the same and exactly on the 1-hour mark, so it's not to control thermal issues.
I have measured mine with an ODB2 and android software and never see the dip. It may be possible it is somehow related to the tool you use to measure. I will have to go back to my data and see if that occurred and I just missed it.

At the end of the charge cycle, the power draw reduces gradually over a 10 minute period. That surprises me. Why did GM engineers go to the trouble to program this gradual decline? Is this perhaps done to approach the maximum cell voltage across all 97 groups slowly? That could be, but the normal charge/discharge currents while driving are 10x higher, so it would not make a difference.
The dip at this end also occurs on my measurements. This is typical of any smart battery charge system. What does not show on your graph is after the dip there is some slight bump in charging or draw which seems to be using the pumps on-board for cooling the battery and on-board charger.
 
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